The present invention relates to detector systems which detect the passage or presence of a vehicle or other object over a defined area. These detector systems are often part of a traffic actuated control system for controlling traffic signal lights.
The detector systems commonly employ an inductive sensor in or near the area to be monitored and sense changes in the sensor's magnetic field to detect the presence or passage of vehicles or other objects. The inductive sensor can take a number of different forms, but commonly is a wire loop which is buried in the roadway and which acts as an inductor.
Known detector systems also include circuitry which operates in conjunction with the inductive sensor to measure the changes in inductance and to provide output signals as a function of those inductance changes. An oscillator circuit connected to the inductive sensor produces a signal having a frequency which is dependent on sensor inductance. The sensor inductance is in turn dependent on whether the inductive sensor is loaded by the presence of a vehicle. The detector system measures changes in inductance of the inductive sensor by monitoring the frequency of the oscillator output signal.
In detector systems known in the art, the detector defines sequential detect cycles. During each detect cycle, cycles of the loop oscillator signal are counted. Concurrently, a second counter measures the duration of a predetermined number of oscillator cycles by counting pulses provided by a very accurate clock pulse source. The measured duration is then compared with a reference duration (whose value is based upon the measured duration during prior detect cycles) and the difference is indicative of a change in oscillator frequency and thus also a change in loop inductance. If the count differs from the reference by at least a threshold amount, the detection system generates a "call" to signal presence of a vehicle.
The detector systems known in the art suffer several disadvantages. First, if the inductive sensor is located near electric power distribution lines, magnetic flux from the power lines can alter the apparent inductance of the loop and therefore the accuracy of the detector system. This fluctuation, which is at the frequency of the power line (60 Hz in the United States), manifests itself as a variation in the value of the measured frequency. Because the measurement period of current vehicle detectors in making a single measurement is usually much shorter than the period of the power line sinusoid, the measured inductance will differ depending upon when the measurement was taken during the cycle of the power line signal. If this condition occurs, and depending on the phase of the power line signal at which the measurement is taken, the variation may be large enough to cause an apparent reduction in sensitivity of the inductive sensor. This can result in false vehicle detections or failure to detect a vehicle entering the detection area. Another drawback is that the vehicle detector may continuously register the presence of a vehicle, even when a vehicle is not present.
Another drawback of known detector systems lies in their mechanism for adapting to compensate for environmental changes which can affect sensor inductance. Commonly, the difference between the measured and reference durations is utilized to modify the reference duration toward the measured duration to thus allow the detector to self-tune or adapt to varying environmental conditions. The reference is modified slowly in response to small deviations or differences between the measured and reference time durations. This mechanism allows the detector to detect vehicles over a relatively long period of time, and under varying environmental conditions.
Although the above described mechanism is adequate for detector systems employing traditional inductive loops, errors arise in systems employing earth's field type inductive sensors ("microloops") as the inductive sensor. In a microloop system, magnetic elements of a vehicle such as stereo speakers, etc. can cause an initial variation in the non-call direction before the transition in the call direction. In a traditional vehicle detector, this initial non-call variation causes the reference to adjust to the maximum level of the initial non-call variation. This premature adaptation of the reference in response to the initial non-call deviation can result in failure to detect the vehicle leaving the area of the microloop, thus resulting in a "locked call" condition.
Another drawback of tradition detector systems is their inability of count multiple vehicles while in "presence mode". In presence mode, the CALL signal is held active for as long as a vehicle is present in the detection area. In known detector systems, this prevents the system from detecting subsequent vehicles entering the detection area while another vehicle is present.
Finally, maintenance on traditional detector systems is often difficult. Several types of faults can occur in a detector system, including shorts, opens, and large changes in inductance. The opens can be caused by shifting ground, cutting of loop wire, corroding contacts or other disturbances of the loop wire. Shorts are cause by moisture and disturbances of the wire. Changes in inductance can by caused by moisture shorting out the turns of the inductive sensor. These faults can come and go because of changes temperature and moisture.
When any of these faults occur, the system will fail to operate properly. Since these faults can come and go, the problem may not be apparent when a technician services the equipment, making corrective maintenance difficult.